This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The spliceosome is the cellular machine responsible for removing the introns that interrupt nearly all human gene transcripts. In a highly dynamic series of molecular interactions, the spliceosome is assembled on each intron from on the order of 150 proteins, as well as 5 structural RNAs. The Jurica group has developed a protocol isolate to spliceosomes assembled in vitro on a synthetic splicing substrate from HeLa cell nuclear extract. LC-MS/MS analysis of these complexes identified a potential "parts list" of over 200 proteins, although the stoichiometric presence of many of these proteins remains to be verified. Control experiments indicate that a subset of the proteins bind the pre-mRNA even in the absence of splicing. 1) In order to further define the composition of the core spliceosome complex Jurica will use mass spectrometry to identify proteins associated with the spliceosome when flanking pre-mRNA is released. 2) They will also compare the composition of spliceosomes arrested at other points of splicing. 3) In addition, they will identify proteins that are exposed on the surface of the spliceosome by chemically modifying purified spliceosome complexes under both native and denaturing conditions. Using mass spectrometry to analyze peptides derived from these samples, they can conclude that peptides modified in both samples are surface exposed, while those modified only in the denatured complexes are buried within the complex. The limiting amount of material and high complexity of their samples will require mass spectrometric instrumentation with very high resolution and very high sensitivity.